In a typical pressurized water reactor (PWR) power plant, various mechanical components and systems are installed on the reactor vessel closure head. These mechanical components and systems include, for example, a control rod drive mechanism (CRDM) cooling system, a reactor vessel closure head lift rig, CRDM seismic restraints, and a CRDM missile shield. Each of these components is typically designed and installed as a permanent fixture to perform designated functions during plant operation. However, during refueling of the reactor these components have to be disassembled in order to remove the reactor vessel closure head from the reactor vessel. These components are stored in designated storage areas, generally inside the reactor containment. Typically, in a PWR plant, a series of steps are followed before the reactor vessel closure head is removed from the reactor vessel. The operational steps that are performed prior to detensioning the reactor vessel closure head studs include some or all of the following:                Remove and store heavy concrete missile shields.        Remove and store the CRDM cooling ducts.        Remove the seismic restraints.        Disconnect and store the CRDM power and rod position indicator cables.        Install the reactor head lifting rig tripod.        Remove cable trays and cables running from the reactor head to the operating deck or walls.        Disconnect heated junction thermocouples, nuclear steam supply system instrumentation, monitoring system cables, and reactor head vent lines.        Install temporary lead shield blankets around the vessel closure head area.        The procedure also requires that the nuts and washers be removed from the reactor vessel closure head and placed in storage racks during preparation for refueling. The storage racks are then removed from the refueling cavity and stored at convenient locations inside containment prior to reactor vessel closure head removal and refueling cavity flooding. The above steps are then reversed while reinstalling the reactor vessel closure head and the related reactor systems.        
Each of these steps contributes significantly to the total cost associated with refueling the reactor. The total costs include costs associated with personnel man-hours required to perform the refueling, power plant down time and consequent loss of electricity production, radiation exposure to personnel, and potential human errors. In addition, the various components that must be removed for refueling activities require a large amount of the limited storage space available inside containment and raise the risk of inadvertent contamination of work and storage areas.
Concepts and designs for integrating some of the reactor vessel closure head systems into a modular integrated head design have been proposed. For example, in U.S. Pat. No. 4,678,623 to Malandra et al., a modular head assembly is disclosed wherein vertical lift rods are attached to the reactor vessel lifting lugs, and a missile shield, seismic support platform, CRDM cooling system, and lift rig are supported by the lift rods above the reactor vessel closure head. Because most or all of the modular head assembly taught by Malandra et al. is supported by the lift rods, however, very large loads are concentrated at the clevis connection at the reactor vessel closure head lifting lugs, which may cause damage to the lifting lugs and/or the body of the reactor vessel closure head. In addition, very heavy components such as the missile shield and the fans are supported at the distal ends of three relatively long lift rods, resulting in an unstable structure that may subject the lift rods to undesirable compressive, bending and torsional stresses. Malandra et al. also does not provide a structure for putting a shroud around the CRDMs.
In U.S. Pat. No. 4,830,814, Altman discloses an integrated head package having a missile shield that is slidably mounted near the distal end of three lift rods connecting to the reactor vessel closure head lifting lugs. A shroud is shown disposed about the CRDMs. Similar to the apparatus disclosed by Malandra et al., however, the heavy missile shield and lifting rig are installed at the distal end of three elongate lift rods that are connected at their proximal end to the reactor vessel closure head lifting lugs. The Altman apparatus, therefore, will also produce relatively high local loads in the reactor vessel lifting lugs and head. Altman also does not disclose any system for cooling the CRDMs.
Some commercial light water reactors—for example, pressurized water reactors produced by Babcock & Wilcox (B&W)—have a reactor vessel closure head having inverted L-shaped flanges that extend upwardly from the reactor vessel closure head. Many B&W reactors also employ a control rod design wherein the lead screw from each control rod must be decoupled from the control rod and parked before the reactor vessel closure head is removed from the reactor vessel. In order to decouple and park the control rod lead screw, a 15-foot tool is typically inserted from above into the CRDM housing. For these types of commercial reactors, therefore, significant overhead space, or headroom, is required above the reactor vessel to accommodate the control rod tool, prior to removing the reactor vessel closure head. To provide the necessary head room, various components disposed above the reactor may need to be disassembled, removed, and stored before the control rod lead screws can be decoupled and parked and the closure head removed.
There is a need, therefore, for an integrated head assembly for a pressurized water reactor that can be removed from the reactor vessel integrally with the reactor vessel closure head, and that does not introduce undue local stresses at the reactor vessel closure head and lifting lugs.